Multilayer semiconductor devices formed on insulating substrates are the foundation for fabrication of integrated circuits. Through a precise series of steps, wafers holding integrated circuits comprising hundreds or millions of active components such as transistors and memory cells are produced. Active devices are those that manipulate a signal, by amplification for example. Integrated circuits also include metal trace conductors that conduct signals between the active components as well as dielectric materials separating the metal traces from one another. The never ending drive for higher bandwidth, processing speeds, greater computing power and memory, and other performance improvements, fuels the need for ongoing design and fabrication of integrated circuits having ever increasing complexity.
The cutoff frequency or speed of microelectronic devices incorporated into integrated circuits is defined in cycles per second, or Hertz (Hz). Cutoff frequency is one of the fundamental parameters in which benchmarks regarding performance improvements are expressed. High frequency systems have in recent years been provided having speeds in excess of one gigahertz (Ghz), meaning that the system can manipulate signals having a commensurate frequency. As the relentless drive for higher system speeds persists, systems operating at 10 Ghz, 100 Ghz or even 1000 Ghz are on the horizon.
In order to function at a given computing speed, a high speed integrated circuit generally must carry a signal that is modulated at a commensurate frequency. Hence, signals carried by a 10 Ghz high speed integrated circuit can be expected to be modulated at such a frequency. As integrated circuit speeds breached 1 Ghz, these signals accordingly entered the frequency range of microwaves, that is, 1–40 Ghz. As integrated circuit speeds continue to escalate, signals will be modulated as millimeter or submillimeter waves. Millimeter waves have a frequency range between about 40–50 Ghz. Submillimeter waves have a frequency range between about 50–1000 Ghz.
High speed integrated circuit signals are typically carried in coplanar metal trace conductors, which may in relatively advanced systems occupy several interconnected planes overlaying each other within the integrated circuit. The coplanar metal trace conductors are located closely together in order to minimize the chip size. The transmission of integrated circuit signals in the coplanar metal trace conductors at frequencies in the range of microwaves, millimeter waves and submillimeter waves can result in secondary propagation of such signals outside of such trace conductors. If such secondarily propagated signals reach adjacent coplanar metal trace conductors, they can interfere with and corrupt other intended circuit signals.
High speed integrated circuit chips are typically made on wafers of a semi insulating substrate such as indium phosphide or gallium arsenide. These disc shaped wafers may be two to twelve inches in diameter and may hold from about a hundred to several thousand individual chips during fabrication. Wafers usually are simultaneously batch processed, in lots of 25, for example. The cumulative investment represented by a single wafer batch during production can be tremendous. Given the complexity of integrated circuits, manufacturing quality control testing is thus an important part of wafer fabrication in order to manage production losses, both during and following completion of such manufacture.
Test transistors are commonly integrated into a wafer next to integrated circuits to facilitate such manufacturing quality control testing. Such test transistors are provided with enlarged input and output contacts for direct application and detection of test signals, but are not otherwise electrically connected with the integrated circuits. Measurement probes are applied to the input and output contacts of such a test transistor. A high frequency signal is introduced into the test transistor through the input contacts, and the transistor output is collected at the output contacts for measurement. If the collected output signal conforms to the performance specifications of the test transistor, correct production of the test transistor is confirmed. Correct test transistor production then constitutes indirect verification of correct production of the integrated circuits on the same wafer.
Test transistor probe contacts must be substantially larger and longer than integrated circuit trace conductors, in order to receive application of a test probe. As the test signal propagates on the wafer surface and within the wafer subsurface, interference with other test transistors or with elements of a nearby integrated circuit can occur, distorting the test signal and invalidating the test results. In testing of integrated circuits themselves having signal frequencies in the range of microwaves, millimeter waves or submillimeter waves during and after manufacture, the same problem of signal propagation outside of the metal trace conductors can occur.
Accordingly, there is a need for high speed integrated circuits and for test transistors that are protected against signal crosstalk in the microwave, millimeter and submillimeter signal frequency ranges. There is also a need for methods of manufacturing integrated circuits and test transistors in a manner allowing provision of such crosstalk protection.